What Does the 1996
National-scale Air Toxics Assessment Mean to Vermont?

To better
assist the public and its understanding of the 1996 National-scale Air
Toxics Assessment, the Vermont Air Toxics Program summarized the results
and general implications of the study. To view a general summary
of the results, please click here.
The estimated risks posed to Vermont's
population are summarized as well.

To view a detailed description
of the uses, sources, chemical properties, and risk characterization of
each of the 33 Hazardous Air Pollutants considered in the NATA, please
click here.

Vermont
General Summary of the 1996
National-scale Air Toxics Assessment

The 1996 National-scale
Air Toxics Assessment (NATA) represents a great step forward in the evaluation
of key air toxic pollutants and their health effects. The US EPA put forth
a tremendous effort into this immense project. Yet, there are some important
issues that must be addressed before the results may be perceived as entirely
credible.

It is difficult to perform
a thorough and reasonable comparison of the 1996 NATA results for Vermont.
Due to the nature of Vermont's industry (i.e., the smaller size), many
of Vermont's stationary sources are included in the federally-classified
"area source" category along with some more typical area sources like
wood stoves, landfills, auto body shops, etc. Therefore it is difficult
for the State to assess the accuracy of EPA's estimates of Vermont's various
area sources. In addition, the Vermont Air Toxics Program feels that the
quality and reliability of some of the emissions data and emission factors
used in the 1996 National Toxics Inventory are questionable.

Vermont has collected ambient
air monitoring data for 21 of the 34
NATA HAPs (see Appendices A & D). Sample results for a several
of the pollutants consistently fall below the method detection limit.
When possible, a monitor-to-model comparison at the census-tract level
was completed. Both a table
and graphical
comparisons were developed to demonstrate the differences. In most cases,
EPA's modeled estimate of the mean annual concentration at a census-tract
level was an order to two orders of magnitude less than the mean
annual monitored concentration. EPA recognizes that "pollutants examined
are typically lower than the measured ambient annual average concentrations
when evaluated at the exact location of the monitors." (Monitors are often
located in area where the highest levels of pollutants are anticipated.)
EPA states that modeled estimates are much closer to monitored concentrations
"when the maximum modeled estimate for distances up to 10-20 km from the
monitoring location are compared to the measured concentrations."

For certain chemicals with
"higher" EPA confidence (e.g., benzene & perchloroethylene), mean
annual modeled concentrations were close to the mean annual monitored
concentrations (Appendices A & B). Additionally, concentrations of
several of the NATA pollutants in Vermont are dominated by regional or
global background concentrations. Examples of these are: benzene, carbon
tetrachloride, chloroform, ethylene dibromide, ethylene dichloride, hexachlorobenzene,
methylene chloride, polychlorinated biphenyls (PCBs), perchloroethylene
(perc), and trichloroethylene. This does not imply, however, that these
pollutants are not emitted in significant quantities in Vermont. In some
cases, they are.

Several NATA results evoke
further questioning.

Unusually
high concentrations of acrolein, acetaldehyde, benzene and 1,3-butadiene
(primarily mobile source related pollutants) were modeled for Orange
County, a very rural Vermont county. There are approximately 55 miles
of interstate highway in the Orange County but Windsor County has more
miles of highway, greater vehicle miles traveled (VMT), but lower modeled
concentrations of these pollutants. Orange County is not densely populated
and does not have many lakes. Emissions from snowmobiles in Orange County
may be somewhat similar to emissions in northeastern Vermont counties.

For
some pollutants dominated by background (carbon tetrachloride, chloroform,
hexachlorobenzene, PCBs), the distribution percentile scales are often
misleading and seem to be very artificial. Some concentration
and emission distribution scales seem to be very artificial as well
and are artifacts of the modeling e.g., hydrazine's 25th
to 50th percentile ranges from 0.000000000063 µg/m3
to 0.0000000013 µg/m3. These levels are below the femtogram
level. This may be statistically correct, but what literal value is
there when percentiles are separated by such small fractions? Where's
the reality in this?

1,1,2,2-tetrachloroethane
has a half-life of > 2 years and is practically inert in the troposphere,
why is the background concentration not elevated above zero? Is it all
assumed to go to the stratosphere?

Vermont
Risk Summary:

According
to EPA's 1996 Median Cancer Risk and Median Noncancer Hazard Quotient
Assessment, of the 33 HAPs assessed, the following pollutants are driving
the risk in Vermont. A full summary of median modeled 1996 cancer risk
and noncarcinogen hazard quotients in Vermont may be viewed at the following
link.

Benzene,
carbon tetrachloride and formaldehyde are driving the 1996 estimated
median cancer lifetime risk in Vermont, according to EPA estimates.
Benzene poses a 10 to 30 in-a-million median cancer risk in Chittenden
County and 3 to 10 in-a-million risk in all other counties. Carbon tetrachloride
poses a 3 to 10 in-a-million risk in the entire state and across most
of the US. (However, as noted above, EPA's background concentration
may lead to an overestimation of risk of 5 in a million.) Modeled risk
for formaldehyde is estimated to be between 3 to 10 in-a-million in
the entire state as well.

Acrolein
is the major noncarcinogenic risk driver in Vermont and the entire U.S.
In Chittenden County, the acrolein hazard quotient is 3 to 10, and in
all other counties, it is 1 to 3.

Note:
A hazard quotient less than one suggests that exposures are likely to
be without an appreciable risk of non-cancer effects during a lifetime.
A hazard quotient greater than one can be best described as only indicating
that a potential may exist for adverse health effects.